Our prior U.S. Pat. No. 5,002,023, the disclosure of which is hereby incorporated by reference, describes a VCT system in which a camshaft is connected to a crankshaft by a chain or belt whose portions between the connected elements do not undergo appreciable length changes during the rotation of the camshaft, notwithstanding the torque pulsations experienced by the camshaft during its normal operation and the resulting pulsations in tension within the chain or belt. However, it is known that camshaft timing relative to a crankshaft can be accomplished by changing the lengths of the various portions of the chain or belt which is trained around the sprockets or pulleys on the camshaft(s) and crankshaft, respectively, and various engine and vehicle manufacturers have a preference for this technique as a technique for accomplishing camshaft phase variations, that is, variations in camshaft position relative to crankshaft position. Prior art chain length variation VCT systems are described, for example, in our prior U.S. Pat. No. 4,862,845 and in U.S. Pat. No. 3,683,875 (Chadwick).
Known prior art chain length variation VCT systems, such as those described in the foregoing references, all require an active, separately actuated device to vary the lengths of the portions of a chain or belt between a sprocket or pulley on a camshaft and a sprocket or pulley on a crankshaft. In an hydraulically operated system utilizing engine lubricating oil as the hydraulic fluid, this can be done by using the engine oil lubricating pump to operate the VCT chain length variation devices, but it adds greatly to the required size of the pump, especially in view of the rapid response times that are required in modern VCT systems.